Evolution Trend of Depth to Groundwater and Agricultural Water-Saving Measure Threshold under Its Constraints: A Case Study in Helan Irrigated Areas, Northwest China

Water conservation is imperative for alleviating water shortages and fostering the establishment of a water-saving society. However, the implementation of water-saving measures caused an increase in the depth to groundwater, resulting in ecological issues. Therefore, reasonable water saving should consider the appropriate depth to groundwater and its evolution trend. This study focuses on the Helan irrigation area in Ningxia as the research subject, aiming to establish thresholds for agricultural water-saving measures while considering constraints by depth to groundwater. The evolution trend of depth to groundwater from 1995 to 2018 was analyzed using the k-Shape cluster and complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) methods. Then, the thresholds of agricultural water-saving measures involved comparing groundwater numerical simulation results for water-saving scenarios, with the control area of ecological groundwater table depth and water shortage rate serving as the evaluation indices. The thresholds for implementing agricultural water-saving measures are as follows: the canal water utilization coefficient is set at 0.62; the proportion of water-efficient irrigation areas to effective irrigation areas should be 40% in the Yellow River irrigation area and 90% in the pumps-up Yellow River irrigation area; additionally, a reduction of 75% in the rice planting area is targeted.